On all newish brass, the nickel coating is VERY thin, and I have NEVER seen any of this thin nickel coating flake.
In fact, I accidentally left a tumbler on for a couple days which contained some nickel plated brass. The nickel plating was almost worn off on some of the cartridges!!!
If anyone has any nickel plated brass that is starting to flake, please post pics. As I said, I have never seen this happen.
It might have happened on very old brass with very thick nickel plating.